Continuing improvements in semiconductor processing techniques have produced a continuing trend in the reduction of feature sizes in integrated circuits (ICs). Smaller transistors have in turn led to smaller power requirements and lower operating voltages. Lower power and low voltage ICs have enabled a broad range of battery powered mobile devices.
Many low power/low voltage devices require both analog and digital circuits. These circuits include amplifiers, digital logic, and interface circuits such as analog-to-digital converters (ADCs) and digital-to-analog converters (DACs).
In general, the optimization of an analog IC requires a different fabrication process than a digital IC. For example, a bipolar process may be preferred for an analog circuit and a CMOS process for a digital circuit. This difference is usually not a problem when a system can be designed using integrated circuits that are exclusively analog or digital. However, for mixed-signal ICs such as ADCs and DACs, both digital and analog circuits are required on the same chip. Examples of analog functions that may be included in a mixed-signal chip are delta sigma modulators, reconstruction filters, and switched capacitor filters. These functions and others often require an amplifier as a building block.
Although a process such as BiCMOS may be used to provide enhanced performance for analog circuits in a mixed-signal IC, BiCMOS is more complex and more expensive than CMOS. Thus, it is often desirable to design an analog circuit using CMOS in order to reduce cost, in spite of the performance limitations. The combination of analog and digital circuits usually entails a process compromise.
Another problem associated with mixed-signal ICs is the noise associated with switching in the digital circuits, and the susceptibility of the analog circuits to the noise. Although design techniques such as cascading of devices may be used improve noise immunity, cascading in circuits with a low supply voltage is constrained by the fact that transistor threshold voltages do not scale down with reduced feature size. A conventional cascode used in a low voltage circuit (e.g. less than 1.5V) may have insufficient voltage headroom.